Evan, 90% alcohol is a good choice. Any of the following cleaners can be used with a plastic scrubbing pad, soft cloth, paper towel...your choice. General consensus is this, and everyone who disagrees or has something to add, please chime in. For track that's new, the alcohol is a good choice. Once it's had time to get a little goop on it from use, a light cleaner like 409 will take off the oil better and then the alcohol will remove any soap film. Other members swear by Goo-Gone, an excellent product to clean with. My personal belief is it needs to be followed by alcohol as it, IMHO, leaves a film that's not quite as conductive as raw metal. For really tough spots, there are Brightboys, a rubber-eraser type product available at most train stores.
DON'TS: 1. Steel wool should never come near your track or your locomotives. Small fragments break off and eventually become one with the magnets in your motor or the gears and bearing-surfaces in your drive train. This will destroy your locomotives.
2. Sandpaper has no place in the process of cleaning track or electrical pickup wheels and rollers. Yes, people use it and it works well. However, it's destruction is of a more insidious nature. Sandpaper scratches. Those micro-scratches make your track, wheels and rollers uneven: what once was polished smooth, to a glass-like finish, now has a multitide of tiny scratches. Those scratches are now the surface that communicates your power to the train.
Consider lightning: it is the gathering of potential power in the earth's surface to finally leap from the highest conductive point from earth to clouds (I know it looks like the reverse, but that's another thread). What does it leave behind, when that massive spark jumps? Carbon---a charred surface where anything present briefly burned.
The goop that accumulates on track and wheels is exactly that: a comination of dust, dirt, oil and the carbon produced by that spark leaping. If your track is scratched, you generate a zillion tiny sparks. If your track is smooth, the metal-to-metal conduction of electricity is uninterupted and you get no sparks. Smooth track = less cleaning. Sandpaper = "my that looks clean!" followed by "Why am I constantly scrubbing this stuff?"
Finally, be careful of anything remotely like a solvent getting close to your plastic. Cars can melt, lettering run, and a bad day suddenly set in big-time if you get careless. If you use extruded pink foam sheets for landscaping, acetone and some paints will dissolve it.
Best wishes!